Method and apparatus for determining the thickness of a dielectric layer

ABSTRACT

The method for determining the thickness of a dielectric layer according to the invention comprises the step of providing an electrically conductive body ( 11 ) having a dielectric layer ( 13 ) which is separated from the electrically conductive body ( 11 ) by at least a further dielectric layer ( 3 ) and a surface ( 15 ) of which is exposed. Onto the exposed surface ( 15 ) an electric charge is deposited, thereby inducing an electric potential difference between the exposed surface ( 15 ) and the electrically conductive body ( 11 ). An electrical parameter relating to the electric potential difference is determined and a measurement is performed to obtain additional measurement data relating to the thickness of the dielectric layer ( 13 ) and/or to the thickness of the further dielectric layer ( 3 ). In this way the thickness of the dielectric layer ( 13 ) and/or of the further dielectric layer ( 3 ) is determined. The method of manufacturing an electric device ( 100 ) comprises this method for determining the thickness of a dielectric layer. The apparatus ( 10 ) for determining the thickness of a dielectric layer is arranged to execute this method.

The invention relates to a method for determining the thickness of adielectric layer comprising the steps of providing an electricallyconductive body having a dielectric layer, a surface of the dielectriclayer being exposed, depositing an electric charge onto the exposedsurface thereby inducing an electric potential difference between theexposed surface and the electrically conductive body, determining anelectric parameter relating to the electric potential difference, andderiving the dielectric layer thickness from the electrical parameter.

The invention further relates to a method for manufacturing an electricdevice in which such a method is performed.

The invention relates further to an apparatus for determining thedielectric layer thickness according to such a method.

WO 02/059631 discloses a method for determining the thickness of adielectric layer as described in the opening paragraph.

In the known method, an electric charge dQ_(C) is deposited onto theexposed surface of the dielectric layer. Due to the deposited electriccharge dQ_(C) an electric potential difference dV between the exposedsurface and the electrically conductive body is induced. An electricalparameter relating to this electric potential difference is determinedby a Kelvin probe or a Monroe probe and subsequently the thickness ofthe dielectric layer is determined from the electrical parameter. Theelectrical parameter may be the electric potential difference dV itselfor alternatively, e.g., a leakage current through the dielectric layer.The leakage current may be time-dependent.

It is a disadvantage of the known method that the thickness of thedielectric layer cannot be determined when the dielectric layer isseparated from the electrically conductive body by at least one furtherdielectric layer.

It is an object of the invention to provide a method which is suited todetermine the thickness of the dielectric layer when the dielectriclayer is separated from the electrically conductive body by at least onefurther dielectric layer.

The invention is defined by the independent claims. The dependent claimsdefine advantageous embodiments.

The invention is based on the insight that in the known method thethickness d₁ of the dielectric layer cannot be determined when thedielectric layer is separated from the electrically conductive body byat least one further dielectric layer because the electrical parameterthen is a function of the thickness d₁ and the thickness d₂ of the atleast one further dielectric layer. In the remainder of this documentthe at least one further dielectric layer is often referred to simply asthe further dielectric layer. When the at least one further dielectriclayer comprises more than one layer, the thickness d₂ comprises thethickness of each of these layers. By performing the measurement,measurement data being a further function of the thickness d₂ areobtained. The measurement data may additionally depend on the thicknessd₁ as well provided that the dielectric layer thickness, which isselected from the thickness d₁ and the thickness d₂, is derivable fromthe electric potential difference and the measurement data. This impliesthat the electrical parameter and the measurement data have differentfunctional dependencies on the thickness d₁ and the thickness d₂ suchthat at least one of the unknowns, i.e. the thickness d₁ and/or thethickness d₂, can be determined from the electrical parameter and themeasurement data.

The measurement may comprise, e.g., a mechanical, optical or electricalmeasurement of the thickness d₁, the thickness d₂ or the thickness d₁plus the thickness d₂.

In the method according to the invention the electrically conductivebody may comprise, e.g., metals, metal alloys, semiconductors or layersof these materials. The dielectric layer and the further dielectriclayer may comprise any type of electrically insulating material such as,e.g., silicon oxide, silicon nitride, tantalum oxide, aluminum oxide,barium strontium titanium or hafnium oxide.

The method according to the invention is further suited for determiningthe thickness d₂ instead of or in addition to determining the thicknessd₁.

The method according to the invention is particularly useful fordielectric layers having a relatively small thickness of, e.g., 10 nm orless. In particular for layers of such small thickness alternativetechniques are often not reliable and/or not accurate.

In an embodiment the dielectric layer has a dielectric constant ε₁, thefurther dielectric layer has a further dielectric constant ε₂, and thedielectric layer thickness, e.g. the thickness d₁, is determined fromthe thickness d₂, the dielectric constant ε₁, the further dielectricconstant ε₂, the electric charge dQ_(C) and the electric potentialdifference dV. Neglecting current leakage through the dielectric layerand the further dielectric layer, dV as a function of dQ_(C), which isalso referred to as the Q-V relationship, is used to derive theequivalent capacitance density C/A, i.e. the capacitance C per area A.From the equivalent capacitance, density C/A the thickness d₁ isderivable by using the formula ε₀ A/C=(d₁/ε₁+d₂/ε₂), where ε₀ denotesthe permittivity of free space. When the further dielectric layercomprises a stack of n layers, where n is an integer larger than one,each of the layers having a thickness d_(i+1) with a dielectric constantε_(i+1), where i is a positive integer smaller than or equal to n, theterm d₂/ε₂ in the above formula is replaced by the sum of all termsd_(i)/ε_(i). When current leakage through the dielectric layer and thefurther dielectric layer cannot be neglected, the electric potentialdifference depends on time. In this case the thickness d₁ can bedetermined from, e.g., the measured electric potential difference as afunction of time by analyzing it analogously to the known method.

Alternatively, in particular in cases where the thickness d₂ is notreadily available, other methods of performing the measurement may beadvantageous which may be based on, e.g., the spectral reflectance ofthe dielectric layer and/or of the further dielectric layer. This methodof determining the thickness d₁ and the thickness d₂ by measuring thespectral reflectance is known from, e.g., U.S. Pat. No. 4,999,509. Itinvolves measuring the spectral reflectance, i.e. the ratio of thereflected light intensity and the incident light intensity as a functionof the wavelength. The measurement data thus obtained are then analyzedusing an optimization procedure the outcome of which at least partlydepends on the initial parameters used in the optimization procedure.Therefore, this technique alone is often not sufficiently reliableand/or accurate, in particular when the dielectric layer and the furtherdielectric layer are relatively thin, e.g. having a thickness of lessthan 50 nm such as, e.g. 2 to 10 nm, or when the further dielectriclayer and the electrically conductive body are separated by anadditional dielectric layer. When this method is used in combinationwith the known method, the electrical parameter imposes a constraintduring the optimization procedure, thereby largely reducing theabove-mentioned dependency on the initial parameters used in theoptimization procedure.

It is often advantageous if after depositing the electric charge dQ_(C)and determining the electrical parameter, and prior to performing themeasurement, the dielectric layer is at least partly removed to expose afurther surface of the further dielectric layer. When the furtherdielectric layer comprises more than one layer, the upper of theselayers, i.e. the layer in direct contact with the dielectric layer, isexposed. In this way it is possible to perform the measurement on theexposed part of the further dielectric layer, i.e. at a position wherethe dielectric layer is absent. As a consequence, the measurement datarelate to a relatively small extent to the thickness d₁, allowing for acomputationally relatively easy determination of the thickness d₁. Whenpartly removing the dielectric layer, it is preferred that themeasurement data relate to the thickness d₁ as little as possible, i.e.not at all.

In this case it is further advantageous if the thickness d₂ of theexposed part of the further dielectric layer remains substantiallyconstant during the step of at least partly removing the dielectriclayer because the measurement data then directly relate to the thicknessd₂. When a part of the further dielectric layer is removed as wellduring the step of at least partly removing the dielectric layer, theremaining exposed further dielectric layer is thinner than the furtherdielectric layer when determining the electrical parameter. For areliable determination of the thickness d₁ this reduction of thethickness d₂ has to be taken into account which complicates theexecution of the method according to the invention.

In many cases it is advantageous if the step of at least partly removingthe dielectric layer comprises an etching step because for many materialcombinations etching recipes are known which allow for selectivelyremoving at least part of the dielectric layer while keeping thethickness d₂ substantially unchanged.

When at least partly removing the dielectric layer to at least partlyexpose the further dielectric layer it is further advantageous ifperforming the measurement comprises the sub-steps of depositing afurther electric charge onto the further exposed surface, therebyinducing a further electric potential difference between the furtherexposed surface and the electrically conductive body, and determining afurther electrical parameter relating to the further electric potentialdifference, the measurement data comprising the further electricalparameter. In this embodiment of the method according to the inventionthe thickness d₂ of the exposed further dielectric layer is determinedin a way analogous to the known method which has the advantage that thethickness d₂ is determined relatively accurately, in particular when thethickness d₂ is relatively small, e.g. below 50 nm. For such arelatively small thickness alternative methods often do not have therequired accuracy.

In another embodiment of the method according to the invention theelectrically conductive body and the further dielectric layer areseparated by an additional dielectric layer, i.e. the dielectric layer,the further dielectric layer and the additional dielectric layerinitially form a stack, and the measurement data relate to a thicknessd₃ of the additional dielectric layer. When leakage currents through thedielectric layer, the further dielectric layer and the additionaldielectric layer can be neglected, the thickness d₁ is readily derivablefrom the formula ε₀ A/C=(d₁/ε₁+d₂/ε₂+d₃/ε₃) where ε₃ denotes thedielectric constant of the additional dielectric layer and A/C is theinverse of the equivalent capacitance density C/A.

It is often preferred that the thickness d₂ and the thickness d₃, ifpresent, are determined as well when executing the method according tothe invention.

Alternatively, in particular in cases where the thickness d₂ and/or thethickness d₃ are not readily available, other methods of performing themeasurement may be advantageous which may be based on, e.g., thespectral reflectance of the dielectric layer and/or of the furtherdielectric layer.

In many cases it is advantageous if performing the measurement comprisesdetermining a spectral reflectance of the exposed surface and/or of afurther exposed surface of the further dielectric layer. This method iswell known in the art and by combining it with the method known from WO02/059631, the ambiguities in analyzing the data by this method arereduced as explained above. The combination of the method known from WO02/059631 with the method of determining the spectral reflectance is inparticular advantageous when the electrically conductive body and thefurther dielectric layer are separated by an additional dielectriclayer. It is often impossible to use the latter method alone on a stackcomprising three or more dielectric layers whereas this is possibleusing the method according to the invention.

In one embodiment, the spectral reflectance data of the entire stack ofthe at least three layers are analyzed using the determined electricalparameter as a constraint in the analysis. In another embodiment thedielectric layer is at least partly removed after having determined theelectrical parameter, the further dielectric layer is at least partlyexposed prior to performing the measurement of the spectral reflectanceof the at least partly exposed further dielectric layer. This latterembodiment has the advantage that the analysis of the spectralreflectance data is relatively easy because it involves only two insteadof three dielectric layers, i.e. only the further dielectric layer andthe additional dielectric layer.

The method for determining the dielectric layer thickness isadvantageous when executing a method for manufacturing an electricdevice comprising an electrically conductive body having a dielectriclayer, the dielectric layer being separated from the electricallyconductive body by at least one further dielectric layer. The method ofmanufacturing an electric device comprises the steps of providing theelectrically conductive body with the at least one further dielectriclayer, providing the at least one further dielectric layer with thedielectric layer, and performing the method for determining thedielectric layer thickness according to the invention for monitoring thesteps of providing the electrically conductive body with the at leastone further dielectric layer and/or of providing the at least onefurther dielectric layer with the dielectric layer.

Many electric devices such as, e.g., transistors have a dielectric layerwhich may be, e.g., a gate dielectric arranged between a semiconductingsubstrate and a gate electrode, or an inter-gate dielectric arrangedbetween a floating gate and a control gate in a non-volatile memorydevice. Often, these dielectric layers comprise two or three, sometimeseven more, separate layers stacked onto each other. Examples are stacksof layers of silicon oxide and silicon nitride, which are often simplyreferred to as an ON layer, and of silicon oxide, silicon nitride andsilicon oxide, which are often simply referred to as an ONO layer. Otherexamples are stacks of layers at least one of which comprises adielectric material having a dielectric constant higher than that ofsilicon dioxide such as, e.g., tantalum oxide, hafnium oxide, zirconiumoxide and aluminum oxide. These materials, which in the art are referredto as high-k materials, often cannot be in direct contact withsemiconductors or metals. Therefore, they are often applied in stackscomprising, e.g., a silicon oxide layer interposed between the high-kmaterial and the electrically conductive body.

To obtain a reliable electric device each of the dielectric layers inthe stack has to have a thickness within a particular range. Duringmanufacture of electric devices the thickness of each of these layershas to be monitored accurately. It is then preferred that the thicknessd₂ and the thickness d₃, if present, are determined as well whenexecuting the method according to the invention.

Performing the method for determining the dielectric layer thicknessaccording to the invention during the manufacture of an electric devicehas the advantage that the dielectric layer, the further dielectriclayer and the additional dielectric layer, if present, can be formedsubsequently in one tool without the need of removing the conductivebody after depositing one of the dielectric layers for measurementpurposes before forming the subsequent dielectric layer.

According to the invention the dielectric layer thickness may bedetermined on a separate electrically conductive body such as, e.g. atest wafer which is processed at the same time in the same chamber asone or more other electrically conductive bodies comprising thepre-fabricated electric device. Alternatively, the dielectric layerthickness may be determined on the same electrically conductive body asthat which comprises the pre-fabricated electric device. The latterembodiment is preferred in a process in which the electricallyconductive bodies are processed one by one, so-called single waferprocessing.

The apparatus for determining the dielectric layer thickness accordingto the method of the invention comprises a charge source for depositingthe electric charge, a measuring device for determining the electricparameter relating to the electric potential difference, and a signalprocessor means for determining the dielectric layer thickness from theelectrical parameter and the measurement data. Preferably, the signalprocessor is arranged to determine the thickness d₁, the thickness d₂and the thickness d₃, if applicable.

These and other aspects of the method and the apparatus for determiningthe thickness of the dielectric layer, and of the method formanufacturing an electric device according to the invention will befurther elucidated and described with reference to the drawings, inwhich:

FIG. 1 is a schematic drawing of a cross-section of the apparatus fordetermining the dielectric layer thickness, and

FIGS. 2A-2D are cross-sections of the electrically conducting body atvarious steps of an embodiment of the method for manufacturing theelectric device.

The Figures are not drawn to scale. In general, identical components aredenoted by the same reference numerals.

The method for determining a dielectric layer thickness comprises thesteps of providing an electrically conductive body 11 shown in FIG. 1which may be, e.g., a silicon wafer, a silicon on insulator wafer or agallium arsenic wafer. The electrically conductive body 11 is held byand electrically connected to a conductive vacuum chuck 18 which iselectrically connected to ground potential. The electrically conductivebody 11 has a dielectric layer 13 which may be composed of any type ofelectrically insulating material such as, e.g., silicon oxide, siliconnitride, tantalum oxide, aluminum oxide, hafnium silicate, zirconiumoxide, lanthanum oxide, praseodium oxide (Pr₂O₃), barium strontiumtitanium or hafnium oxide. The dielectric layer 13 is separated from theelectrically conductive body 11 by at least one further dielectric layer3 which may be composed of any type of electrically insulating materialsuch as, e.g., the materials mentioned above in relation to dielectriclayer 13. The dielectric layer 13 has a surface 15 which is exposed.

In a step of the method according to the invention an electric chargedQ_(C) is deposited onto the exposed surface 15, thereby inducing anelectric potential difference dV between the exposed surface 15 and theelectrically conductive body 11. The electric potential difference dV isa function of the thickness d₁ of the dielectric layer 13 and thethickness d₂ of the further dielectric layer 3.

The apparatus 10 for determining the dielectric layer thickness shown inFIG. 1 is similar to that shown in FIG. 1 of WO 02/059631. It comprisesa charge source 16 for depositing the electric charge dQ_(C). The chargesource 16 may be, e.g., a corona discharge source including a coronacharging wire 14 a which receives a high voltage potential of either apositive or negative polarity and a corona-confining electrode ring 14b, e.g., a metal ring, held at ground potential or a bias. Preferably,the charge source 16 is able to deposit the charge dQ_(C) uniformly onthe surface 15 of the dielectric layer 13, preferably within a radius ofabout 6 to about 10 mm. The corona discharge source is able to produce acontrolled ionic discharge flux (ionic) current suitable for depositingthe electric charge dQ_(C). The flux may be adjustable and range, e.g.,from about 10⁻⁶ to about 5×10⁻⁶ A cm⁻².

Preferably, the amount of the deposited charge dQ_(C) is relativelysmall, e.g. determined by the above-specified flux which is depositedduring less than, e.g., 30 seconds. In this way the amount of chargedtraps generated in the dielectric layer 13 and the further dielectriclayer 3 due to the current leakage is reduced which enhances theaccuracy of the method. The amount of the electric charge dQ_(C)deposited may be controlled by adjusting the high voltage potential, theheight of the corona electrode above the wafer, and/or the bias voltageapplied between the corona charging wire 14 a and the ion flux confiningelectrode 14 b. The charge source 16 is able to deposit either apositive or a negative charge on the surface 15. Preferably, the chargesource 16 charges the surface 15 with a positive corona dischargebecause a negative corona discharge is more difficult to control withrespect to charging uniformity.

In a subsequent step an electric parameter relating to the electricpotential difference dV due to the deposition of the electric chargedQ_(C) is determined. To this end the apparatus 10 further comprises ameasuring device 22 for determining the electrical parameter relating tothe potential difference dV. The potential measuring device 22 may be,e.g., a Kelvin probe or a Monroe-type probe. It is able to determine theelectric potential difference dV by measuring the contact potential ofdielectric layer 13 with respect to a reference electrode 30. Sensors ofthese types are described, e.g., in the references stated in WO02/059631, page 9, line 28-31. Typically, the electrode 30 is separatedfrom the top surface of the dielectric film 13 by an air gap of about afraction of about a millimeter. Charge source 16 and measuring device 22are spaced apart from each other on a mount at a fixed distance x₀ of,e.g., 2 cm, between their centers. After depositing charge dQ_(C) onsurface 15, solenoid 20 is used to translate charge source 16 andmeasuring device 22 by a distance x₀ such that the measuring device 22is above the surface 15 previously provided with the electric chargedQ_(C).

When the electrically conductive body 11 is a semiconductor, the changeof the contact potential V measured by the vibrating Kelvin or Monroeelectrode is not solely determined by the electric potential differencedV caused by the deposited electric charge dQ_(C), but is equal to thechange in voltage drop across the dielectric layer dV plus the change inthe semiconductor surface barrier, V_(SB), i.e. V=dV+V_(SB). Preferably,the apparatus 10 further comprises light sources 23, 25, preferablygreen or blue light emitting diodes, to illuminate testing site 15during charging (light source 23) and during measuring (light source25), thereby reducing the value of V_(SB) by collapsing the surfacedepletion region in case the electrically conducting body 11 is asemiconductor.

The apparatus 10 further comprises a signal processing device 12 whichis a computer arranged to receive a signal relating to the electricpotential difference. The signal processing device 12 is furtherarranged to control the high voltage of the charge source 16 and thesolenoid 20.

After determination of the electrical parameter relating to the electricpotential difference dV, the dielectric layer 13 is removed to expose afurther surface of the further dielectric layer 3 by an etching step. Inone embodiment the dielectric layer 13 is composed of silicon nitride,the further dielectric layer 3 is composed of silicon dioxide and theetching step comprises a wet etch in which phosphoric acid, H₃PO₄, isused as the etching agent. Such an etching process is selective towardssilicon nitride, i.e. silicon nitride is removed more effectively thansilicon dioxide. The selectivity is improved by heating the etchingagent and is usually greater than 25:1. Therefore, the thickness of theexposed part of the further dielectric layer is kept substantiallyconstant during the step of at least partly removing the dielectriclayer.

The electrically conductive body 11 thus obtained has only the furtherdielectric layer 3. In a subsequent step of the method according to theinvention a measurement is performed for obtaining measurement databeing a further function of the thickness of the at least one furtherdielectric layer 3. Performing the measurement comprises the sub-stepsof depositing a further electric charge dQ_(CF) onto a further exposedsurface of the further dielectric layer 3, thereby inducing a furtherelectric potential difference dV_(F) between the further exposed surfaceand the electrically conductive body 11 using the apparatus 10 in ananalogous way, followed by determining a further electrical parameterrelating to the further electric potential difference dV_(F).

The further electric charge dQ_(CF) is deposited by charge source 16 andthe further electric potential difference dV_(F) is determined by themeasuring device 22 in way analogous to that described above. The signalprocessing device 12 is further arranged to receive a further signalrelating to the measurement data, and to determine the dielectric layerthickness from the signal and the further signal.

In this embodiment the electrical parameter comprises the electricpotential difference dV, the measurement data comprises the furtherelectric potential difference dV_(F), and the dielectric layer thicknessis derived from the electrical parameter and the measurement data, saiddielectric layer thickness being selected from the thickness d₁ of thedielectric layer 13 and the thickness d₂ of the at least one furtherdielectric layer 3. The dielectric layer 13 has a dielectric constantε_(l), the further dielectric layer 3 has a further dielectric constantε₂, and the thickness d₁ of the dielectric layer 13 is determined fromthe thickness d₂ of the further dielectric layer 3, the dielectricconstant ε₁, the further dielectric constant ε₂, the electric chargedQ_(C) and the electrical potential difference dV. To this end, thethickness d₂ of the further dielectric layer 3 is determined from thefurther electric potential difference dV_(F) and the further electriccharge dQ_(CF) according to the formula go A/C_(F)=d₂/ε₂, where ε₀denotes the permittivity of free space and A/C_(F) is the inverse of theequivalent capacitance density derived from dV_(F) as a function ofdQ_(CF). Subsequently, the thickness d₁ of the dielectric layer 13 isdetermined according to the formula ε₀ A/C=(d₁/ε₁+d₂/ε₂), where A/C isthe inverse of the equivalent capacitance density derived from dV as afunction of dQ_(C). The signal processing device 12 is arranged to solvethese two formulas so as to provide the thickness d₁ and the thicknessd₂.

In an alternative embodiment of the method, performing the measurementcomprises determining a spectral reflectance of the exposed surface 15of the dielectric layer 13. Alternatively, the spectral reflectance maybe determined of the further exposed surface of the further dielectriclayer 3. In these cases the thickness d₂ is determined from the measuredspectral reflectance in a way analogous to U.S. Pat. No. 4,999,509.

In another embodiment the electrically conductive body 11 and thefurther dielectric layer 3 are separated by an additional dielectriclayer 33 shown in, e.g., FIG. 2A. The method comprises the steps ofdepositing the electric charge dQ_(C) on the exposed surface of thedielectric layer 13, determining the electrical parameter relating tothe electric potential difference dV, partly removing the dielectriclayer 13 to expose the further dielectric layer 3, which results in thestructure of FIG. 2B, depositing the further electric charge dQ_(CF) onthe exposed surface of the further dielectric layer 3, and determiningthe further electrical parameter relating to the further electricpotential difference dV_(F), analogous to the embodiment of the methoddescribed above. The method further comprises the steps of partlyremoving the further dielectric layer 3 to expose the additionaldielectric layer 33, which results in the structure of FIG. 2C,depositing an additional electric charge dQ_(CA) on the exposed surfaceof the additional dielectric layer 33, and determining an additionalelectrical parameter relating to the additional electric potentialdifference dV_(A). The measurement data comprise the further electricpotential difference dV_(F) and the additional electric potentialdifference dV_(A). The measurement data are a further function of thethickness d₃ of the additional dielectric layer 33. The dielectric layerthickness is selected from the thickness d₁, the thickness d₂ and thethickness d₃, and is derived from the electrical parameter and themeasurement data via the following formulas: ε₀ A/C_(A)=d₃/ε₃,A/C_(F)=(d₂/ε₂+d₃/ε₃) and ε₀ A/C=(d₁/ε₁+d₂/ε₂+d₃/ε₃), where ε₃ denotesthe dielectric constant of the additional layer 33. Here, A/C is theinverse of the equivalent capacitance density derived from dV anddQ_(C), A/C_(F) is the inverse of the equivalent capacitance densityderived from dV_(F) and dQ_(CF), and A/C_(A) is the inverse of theequivalent capacitance density derived from dV_(A) and dQ_(CA).

In an alternative embodiment of the method, performing the measurementcomprises determining a spectral reflectance of the exposed surface 15of the dielectric layer 13. Alternatively, the spectral reflectance maybe determined of the further exposed surface 15′ of the furtherdielectric layer 3 and/or of the additional exposed surface 15″ of theadditional dielectric layer 33 shown in FIGS. 2B and 2C, respectively.In these cases the thickness d₂ and/or the thickness d₃ are determinedfrom the measured spectral reflectance in a way analogous to U.S. Pat.No. 4,999,509.

The method of manufacturing an electric device 100 according to theinvention comprises the steps of providing the electrically conductivebody 11 with the further dielectric layer 3, providing the furtherdielectric layer 3 with the dielectric layer 13, and performing themethod for determining the dielectric layer thickness according to theinvention. In the embodiment shown in FIGS. 2A-2D the electricallyconductive body 11 is a monocrystalline silicon wafer which prior toproviding the further dielectric layer 3 is provided with the additionallayer 33. After having provided the dielectric layer 13 the exposedsurface is provided with a layer of polycrystalline silicon which issubsequently patterned to form the gate layer 2 shown in FIG. 2A.Subsequently, the method for determining the dielectric layer thicknessas described above is performed for monitoring the steps of providingthe electrically conductive body 11 with the further dielectric layer 3and/or of providing the further dielectric layer 3 with the dielectriclayer 13. To this end the charge dQ_(C) is deposited on surface 15 oflayer 13 shown in FIG. 2A and the resulting electric potentialdifference dV is determined. Subsequently, part of the dielectric layer13 is removed to expose surface 15′ of the further dielectric layer 3,shown in FIG. 2B, and the further charge dQ_(CF) is deposited on surface15′ and the resulting further electric potential difference dV_(F) isdetermined. In a next step part of the further dielectric layer 3 isremoved to expose surface 15″ of the additional dielectric layer 33,shown in FIG. 2C, and the further charge dQ_(CA) is deposited on surface15″ and the resulting further electric potential difference dV_(A) isdetermined. Finally, the additional dielectric layer 33 is partlyremoved to expose the electrically conducting body 11, and adjacent tothe stack thus formed a source region 50 and a drain region 51 areformed by implanting ions.

The electric device 100 obtained in this way and shown in FIG. 2D is atransistor. It comprises an electrically conductive body 11 having thedielectric layer 13, the dielectric layer 13 being separated from theelectrically conductive body 11 by at least a further dielectric layer3.

In summary, the method for determining the thickness of a dielectriclayer according to the invention comprises the step of providing anelectrically conductive body 11 having a dielectric layer 13 which isseparated from the electrically conductive body 11 by at least a furtherdielectric layer 3 and a surface 15 of which is exposed. Onto theexposed surface 15 an electric charge is deposited, thereby inducing anelectric potential difference between the exposed surface 15 and theelectrically conductive body 11. An electrical parameter relating to theelectric potential difference is determined and a measurement isperformed to obtain additional measurement data relating to thethickness of the dielectric layer 13 and/or to the thickness of thefurther dielectric layer 3. In this way the thickness of the dielectriclayer 13 and/or of the further dielectric layer 3 is determined. Themethod of manufacturing an electric device 100 comprises this method fordetermining the thickness of a dielectric layer. The apparatus 10 fordetermining the thickness of a dielectric layer is arranged to executethis method.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

1. A method for determining a dielectric layer thickness, the method comprising the steps of: providing an electrically conductive body having a dielectric layer, the dielectric layer being separated from the electrically conductive body by at least one further dielectric layer, a surface of the dielectric layer being exposed, depositing an electric charge onto the exposed surface, thereby inducing an electric potential difference between the exposed surface and the electrically conductive body, the electric potential difference being a function of a thickness of the dielectric layer and a thickness of the at least one further dielectric layer, determining an electrical parameter relating to the electric potential difference, and performing a measurement for obtaining measurement data being a further function of the thickness of the at least one further dielectric layer, the dielectric layer thickness being selected from the thickness of the dielectric layer and the thickness of the at least one further dielectric layer, and deriving the dielectric layer thickness from the electrical parameter and the measurement data.
 2. A method as claimed in claim 1, wherein the dielectric layer has a dielectric constant, the further dielectric layer has a further dielectric constant, and the dielectric layer thickness is determined from the dielectric constant, the further dielectric constant, the electric charge, the electrical parameter, and the thickness of the dielectric layer or the thickness of the at least one further dielectric layer.
 3. A method as claimed in claim 1, wherein after the step of determining the electrical parameter and prior to the step of performing the measurement, the method further comprises the step of at least partly removing the dielectric layer for exposing a further surface of the at least one further dielectric layer.
 4. A method as claimed in claim 3, wherein the thickness of the exposed part of the at least one further dielectric layer is kept substantially constant during the step of at least partly removing the dielectric layer.
 5. A method as claimed in claim 3, wherein the step of at least partly removing the dielectric layer comprises an etching step.
 6. A method as claimed in claim 3, wherein the step of performing the measurement comprises the sub-steps of: depositing a further electric charge onto the exposed further surface, thereby inducing a further electric potential difference between the further exposed surface and the electrically conductive body, and determining a further electrical parameter relating to the further electric potential difference, the measurement data comprising the further electrical parameter.
 7. A method as claimed in claim 1, wherein the electrically conductive body and the further dielectric layer are separated by an additional dielectric layer, the measurement data being a further function of a thickness of the additional dielectric layer, the dielectric layer thickness being selected from the thickness of the dielectric layer, the thickness of the further dielectric layer and the thickness of the additional dielectric layer, the dielectric layer thickness being derivable from the electric potential difference and the measurement data.
 8. A method as claimed in claim 3, wherein the step of performing the measurement comprises the step of determining a spectral reflectance of the exposed surface and/or of the further exposed surface.
 9. A method of manufacturing an electric device the electric device comprising an electrically conductive body having a dielectric layer, the dielectric layer being separated from the electrically conductive body by at least one further dielectric layer, the method comprising the steps of: providing the electrically conductive body with the at least one further dielectric layer, providing the at least one further dielectric layer with the dielectric layer, and performing the method for determining the dielectric layer thickness as claimed in claim 1 for monitoring the steps of providing the electrically conductive body with the at least one further dielectric layer and/or of providing the at least one further dielectric layer with the dielectric layer.
 10. An apparatus for determining the dielectric layer thickness according to the method as claimed in claim 1, the apparatus comprising: a charge source for depositing the electric charge, a measuring device for determining the electrical parameter relating to the electric potential difference, and a signal processing means for determining the dielectric layer thickness from the electrical parameter and the measurement data. 